36. In summer squash, white colour fruit (W) is dominant over yellow colour (w) and disc-shaped phenotype (D) is dominant over sphere-shaped phenotype (d).
Determine the genotype of the parents if the cross between white, sphere crossed with white, sphere gives 1/4 white, sphere and 14 yellow, Sphere.
(1) WWDD X wwdd    (2) Wwdd X Wwdd
(3) WwDd X wwdd     (4) WWDD x WWdd

Interpreting the phenotypes

Gene definitions

• Color locus: W = white (dominant), w = yellow (recessive)
• Shape locus: D = disc (dominant), d = sphere (recessive)

Parents: both white, sphere

Phenotypically: Color W– (white), Shape dd (sphere)

Cross outcome: 1/4 white sphere, 1/4 yellow sphere (all progeny sphere = dd)

Shape analysis

Only sphere progeny → both parents must be dd

Color analysis

1:1 white:yellow ratio among sphere progeny → Ww × Ww

Ww × Ww → 3/4 white (WW + Ww), 1/4 yellow (ww)

Full parental genotypes

Both parents: Wwdd (white, sphere)

Expected full ratios:

• White sphere (WWdd + Wwdd): 3/4
• Yellow sphere (wwdd): 1/4

Observed 1/4 + 1/4 matches portion of this distribution [web:109].

Option analysis

• WWDD × wwdd: Parents white disc × yellow sphere (wrong phenotypes)
• Wwdd × Wwdd: correct – both white sphere parents
• WwDd × wwdd: First parent disc-shaped (wrong)
• WWDD × WWdd: No yellow offspring possible

Therefore, parental genotypes are Wwdd × Wwdd (option 2).